未知题型 Commercials create che sensation of loudness because____________________.A.TV stations always operate at the highest sound levelsB.their sound levels are kept around peak levelsC.their sound levels are kept in the middle frequency rangesD.unlike regular programs their intensity of sound varies over a wide range

未知题型 SQL数据定义语言的主要作用是创建存储数据的结构，而数据操纵语言的主要作用则是向数据库中填写数据，具体包括_________、删除、修改等操作。

单项选择题 A.N(1,4)B.N(0,1)C.N(1,1)D.N(1,2)